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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

TIN-PLATE

255

slightly alkaline with ammonia, treated with 45-50 c.c. of ammonium
sulphide 1 prepared from ammonia of D = 0-910, cooled, made up to volume
and filtered through a dry filter. The alkaline sulphide solution serves
for the determination of the tin and the precipitate, consisting of lead and
iron sulphides, for determining the lead.

A. DETERMINATION OF THE TIN. i. Electrdytically. Either stationary
or rotating electrodes may be used.

(a] Rotating electrodes. 100 c.c. of the filtrate are treated in a
300 c.c. beaker with 20-30 c.c. of 40% sodium sulphite solution to
reduce the polysulphides, made up to about 200 c.c. and electrolysed at
60. Coppered and then tinned Winkler cathode z ] spiral anode, 1000
revs, per minute ; temperature, 60 ; ND100, 3-3-5 amp. ; voltage, 5-6 ;
duration, T-5--20 minutes.

When the electrolysis is complete, the rotation of the anode is sus-
pended and, without interrupting the current, the electrolytic beaker replaced
by one filled with water ; after some time, the cathode is detached, washed
successively with water, alcohol and ether, dried at 70 and weighed. The
weight found, multiplied by 3, gives the tin "contained in the sample taken.

When rotating electrodes are used, the determination of tin in tin-plate
occupies not longer than an hour.

(6) Stationary electrodes. In a |-litre conical flask, 100 c.c. of the
alkaline sulphide filtrate are treated, under a draught hood, with a little
potassium cyanide to reduce the polysulphides, and hydrochloric acid
diluted with an equal volume of water added until the reaction is acid, the
flask being kept covered with a funnel and shaken meanwhile. When
evolution of gas ceases, the liquid is boiled for some time with 25 c.c. of
cone, hydrochloric acid to dissolve the tin sulphide which separates and
to coagulate the sulphur. After addition of 25 grams of ammonium oxalate
and dilution to 250 c.c., the liquid is electrolysed: coppered Winkler cathode",
Winkler spiral anode ; ND100 == i amp. j voltage = 3-4 ; temperature,
60 ] duration (for 0-1-0-5 gram Sn), 4-5 hours (see also Electrolytic Deter-
mination of Tin in Ordinary Bronzes). Multiplication by three of the tin
found gives the amount in the sample taken.

2. Grav{metrically. In a J-litre conical flask, 50 or 100 c.c. of the alkaline
sulphide filtrate (see i, p. 254) are diluted with water and acetic acid added
until the reaction is faintly acid, the tin being thus precipitated as sulphide.
After standing overnight, the precipitate is filtered off, washed with 10%
ammonium acetate solution and dried at 100-120. The filter and precipitate
are introduced into a tared porcelain crucible, the filter-paper incinerated
with addition of a few crystals of ammonium carbonate and then ignited
until the stannic sulphide is completely converted into the white oxide
and weighed : Sn02 X 0-7881 = Sn.

1  In presence of copper it is convenient to use sodium sulphide, but in such case
the rapid determination of the tin with rotating electrodes cannot be carried out.

2  For the coppering of the electrode, see p. 225.    For the subsequent tinning, the
following solution is employed : 3-4 grams of stannic ammonium chloride are dissolved
in 150 c.c. of cold water to which are added 150 c.c. of saturated ammonium bioxalate
solution.    The cathode is immersed in this solution and the current passed for some
minutes :   NDJOO =0-2-0-5 ampere.which thetreated with an excess